I find when I'm writing that one of the most fundamental things I can do with my world building is creating belief systems. This applies to whether the characters in the story accept that system or not. It shapes who they are in the world. And most of all, I like doing it. I love creating a mythos for the world, something that characters either subscribe to, or reject.
What got me thinking about this was working on an upcoming series I have. But since that book isn't out yet, I'm going to focus on a two others. Starting with The Frozen Queen. In this series, Tech has become a magical part of religion. People believe that priests are performing miracles because technology is denied them. They are taught that their Queen is divine and that she is the voice of the Gods of Terra on New Terra. This has a profound impact on Phaedra, both who she is and where she's going. I honestly don't believe she'd be the same character without this. Phaedra has a strong objection to this religion for various reasons and it has a profound effect on what she wants. I don't want to spoil the book, but some of her decisions are based on this belief system and how it effects her and those around her.
The belief system in the Rise of the Hawk series is more complex and has a profound effect on people dependent on how they are educated. The foundation of the belief system maintaining a Balance of the Four Sides of a complete person. There is the Warrior (which represents the Physical nature of a person) the Mother (who represents the Nurturing side of a person) the Artist (or Creative side) and finally the Scholar (or Intellectual side). Different characters have different views on the Four Sides and the pursuit of them. A Purist, for instance, seeks to pursue the study of all Four Sides and balances those four parts within them. Then there are those who pursue the studies of one or more depending on what they see as important. Some will study all Four Sides, but lean heavily towards one or two. The most profound impact of this religion is that on this world it has gone from a pursuit to becoming a complete person to those deifying each personification. Of course, there are those who think the pursuit is useless as well. The most obvious case in point are the three sisters Maevan, Korla, and Renna. While all three girls study the Four Sides, Maevan and Korla lean heavily towards the way of the Warrior whereas Renna leans towards the Artist. While Renna studies as required, as she develops it becomes obvious that she doesn't feel the same connection to the Warrior that Maevan and Korla do. While Maevan still tries to find the Balance, Korla is more strongly identified with the Warrior and the Mother. This difference comes out in their fighting styles. While Maevan is more refined and follows classical forms identified with her weapon of choice, Korla is brutal in combat. This brutality comes from an intense desire to protect those who she loves. Maevan's need to protect is extended beyond family and encompasses her entire people.
My point is this. When you build a world and put people in it, you need to know who these people are and a significant portion of that is what they believe. In an upcoming series, my character takes this a step beyond where even though according to her beliefs what she does condemns her, her love for those important to her supersedes her need for a trouble free afterlife. Would she react the same way to her situation if she didn't have her beliefs in place? I don't think so.